Sunday, July 4, 2010

Treating Bronze Objects

Many bronzes have been recovered in excavations at Gordion. The large amount of bronze material, and the moisture in the burial and storage environments result in corrosion on many of these objects. The application of Benzotriazole (BTA), a corrosion inhibitor, is a common treatment for these materials. This is a less invasive treatment options than those often employed in the past, such as chemical stripping.

The objects requiring treatment were selected in the course of our condition survey because of visible active corrosion. The presence of active corrosion often indicates that environmental conditions are affecting the stability of the objects. Past research has been done at Gordion to examine the conditions within the display cases, including Oddy tests and monitoring with dataloggers. The results of this research indicate that these objects are displayed in relatively stable conditions. It now seems likely that the corrosion could be caused by chemical reactions from previous treatments. There are not written records about many of the early conservation treatments, but personal recollections of past practices confirm that this is a possibility.

The standard procedure for treating bronze objects at Gordion follows these general procedures:

If necessary, objects are desalinated to remove soluble salts and 
chemical residues from previous treatments.  

The objects are mechanically cleaned, using a glass bristle brush and
scalpel, and then brushed with ethanol. 

They are then degreased with acetone and placed into a solution of BTA in 
ethanol. The objects are placed into a vacuum chamber to help increase
 the penetration of the solution.

After removal from this chamber, the objects are coated with dilute B-44 acrylic resin. Here we are coating the larger bronze object after treatment with BTA by filling a plastic bag with B-44 solution. This allows us to evenly coat the surface using a minimal amount of resin.

Recently there has been discussion among conservators about the appropriate procedures and effectiveness of BTA as a treatment for bronze corrosion. Julie Unruh, Gordion conservator-at-large, has been actively researching these issues. Questions include the effectiveness of BTA and AMT (another corrosion inhibitor) in combination, the appropriate length of time for BTA submersion, the efficacy of the vacuum chamber, and the possible substitution of corrosion inhibitors with desiccation and environmental controls. If anyone wants to share their knowledge of the effectiveness of these treatments, we would be very happy to hear your thoughts. 

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